It is necessary to accurately perform an air-fuel ratio control (i.e., a fuel injection control) in order to satisfy severe law regulations relating to purification of exhaust gas. In order to perform accurate air-fuel ratio control, it is necessary to accurately calculate an air quantity charged into an engine cylinder (i.e., a cylinder charged air quantity) and appropriately set a fuel injection quantity in accordance with the cylinder charged air quantity.
In order to raise the accuracy for calculating the cylinder charged air quantity, data of the upstream side pressure of a throttle valve (hereafter referred to as “throttle upstream pressure”) is required. In this regard, in a naturally aspirated engine, the throttle upstream pressure is equivalent to an atmospheric pressure. However, in the case of an engine with a supercharger, in an operation range where a charged pressure of a supercharger is generated, the throttle upstream pressure becomes higher than an atmospheric pressure due to the charged pressure.
Conventionally, an internal combustion engine is provided with a pressure sensor to detect a throttle upstream pressure, which however increases its manufacturing cost.
Among engines with superchargers, as disclosed in JP-2006-27763A and JP-2006-194107A (EP-1837512A1), a throttle upstream pressure (compressor downstream pressure) is calculated by using a compressor model (turbocharger model) in which supercharging effects by a compressor of the supercharger are modeled.
As disclosed in JP-2002-201998A (U.S. Pat. No. 6,497,214B2), a cylinder charged air quantity is calculated by using an equation of an intake system model made by modeling behaviors of an intake air during a period where a change in a throttle opening degree causes a change in an actual cylinder charged air quantity. In such an internal combustion engine, in order to reduce a CPU load for engine control by simplifying the calculation of a throttle flow rate parameter, a map (table) of the throttle flow rate parameter using a throttle opening degree as a parameter is stored in ROM in advance, and a throttle flow rate parameter corresponding to a present throttle opening degree is read by searching the map.
A compressor model (turbocharger model) has many adaptation subjects. Therefore, there is required a lot of work time for the adaptation. Further, there exists great influence of manufacture dispersion of the adaptation subjects, decreasing dispersion of estimated values of the throttle upstream pressure. Further, due to manufacture dispersion or aging of parts (throttle valve etc.) of the intake system, a throttle opening sensor, etc., there occurs a difference between a map value (adaptation value) of the throttle flow rate parameter and an actual value of the throttle flow rate parameter in an actual vehicle, decreasing the calculation accuracy of the cylinder charged air quantity.